FIG. 1 shows a typical representation of a switching power supply according to the prior art. The switching power supply 100 shown in FIG. 1 is made up of an input power source (typically a direct current (DC) voltage) 101, an electromagnetic interference filter (EMI filter) 102, an input filtering capacitor 103, a polarity detector 104, an inrush current limiter 105, a switch device 106, and a DC-DC converter 107. The electromagnetic interference filter 102 is coupled to the input power source 101 and is used for suppressing the differential mode noise or the common mode noise resulted from the circuit. The input filtering capacitor 103 is coupled to the input power source 101 and is used for smoothing the input DC voltage supplied from the input power source 101. The switch device 106, which is coupled to the input filtering capacitor 103 and is normally implemented by a cascaded switching transistor circuit, is used for alternately coupling the input DC voltage to the DC-DC converter 107. The DC-DC converter 107 is coupled to the switch device 106 and is used for converting the input DC voltage into one or more predetermined DC voltages to meet a set of specifications. The polarity detector 104 is used for detecting the change of the polarity of the input DC voltage and preventing the input DC voltage from being connected in reverse polarity. The inrush current limiter 105 is used for prohibiting the inrush current from proliferation.
In normal operation, the input filtering capacitor 103 is fully charged in order to remove the high-frequency harmonics of the input DC voltage. However, if the input terminal of the switching power supply is short-circuited, the input filtering capacitor 103 starts discharging its stored energy, and a large reverse current directing from the input filtering capacitor 103 to the input terminal of the switching power supply is circulating via the internal circuit components located in the proximity of the input terminal, for example, the electromagnetic interference filter 102, the polarity detector 104, the inrush current limiter 105, and the switch device 106. These internal circuit components would be vulnerable to the surging of the reverse current, and would be further burnt down if the discharging loop of the reverse current does not break off instantaneously.
Therefore, in case of the short circuit occurring to the input terminal, the energy stored in the input filtering capacitor is discharged to the input side of the switching power supply, and the internal circuit components would fail without the protection of a safeguard device.
There is a tendency to develop a reverse current protection circuit that can protect the internal circuit components of a switching power supply from damage as a result of a reverse current stemming from an input filtering capacitor when the input terminal is short-circuited.